Sheet molding compound flow field plate, bipolar plate and fuel cell

ABSTRACT

This invention provides a highly electrically conductive sheet molding compound (SMC) composition and a fuel cell flow field plate or bipolar plate made from such a composition. The composition comprises a top sheet, a bottom sheet, and a resin mixture sandwiched between the top sheet and the bottom sheet. At least one of the top sheet and bottom sheet comprises a flexible graphite sheet, which has a substantially planar outer surface having formed therein a fluid flow channel. Further, the resin mixture comprises a thermoset resin and a conductive filler present in a sufficient quantity to render the flow field plate electrically conductive enough to be a current collector (preferably with a conductivity no less than 100 S/cm). Preferably, both the top and bottom surfaces are flexible graphite sheets, each having a substantially planar outer surface having therein a fluid flow channel formed by embossing. These two flexible graphite sheets are well-bonded by the middle resin mixture layer to form a highly conductive bipolar plate, which is particularly useful for proton exchange membrane fuel cell applications.

The present invention is based on the research results of a projectsupported by the NSF SBIR-STTR Program. The U.S. government has certainrights on this invention.

FIELD OF THE INVENTION

The present invention relates to a sheet molding compound (SMC) for usein a fuel cell bipolar plate or flow field plate. In particular, itrelates to a flexible graphite-based, highly electrically conductive SMCfor use as a flow field plate or bipolar plate in a proton exchangemembrane fuel cell.

BACKGROUND OF THE INVENTION

A fuel cell converts chemical energy into electrical energy and somethermal energy by means of a chemical reaction between a fuel (e.g.,hydrogen gas or a hydrogen-containing fluid) and an oxidant (e.g.,oxygen). A proton exchange membrane (PEM) fuel cell uses hydrogen orhydrogen-rich reformed gases as the fuel, a direct-methanol fuel cell(DMFC) uses methanol-water solution as the fuel, and a direct ethanolfuel cell (DEFC) uses ethanol-water solution as the fuel, etc. Thesetypes of fuel cells that require utilization of a PEM layer as a protontransport electrolyte are collectively referred to as PEM-type fuelcells.

A PEM-type fuel cell is typically composed of a seven-layered structure,including (a) a central PEM electrolyte layer for proton transport; (b)two electro-catalyst layers on the two opposite primary surfaces of theelectrolyte membrane; (c) two fuel or gas diffusion electrodes (GDEs,hereinafter also referred to as diffusers) or backing layers stacked onthe corresponding electro-catalyst layers (each GDE comprising porouscarbon paper or cloth through which reactants and reaction productsdiffuse in and out of the cell); and (d) two flow field plates (or abi-polar plate) stacked on the GDEs. The flow field plates are typicallymade of graphite, metal, or conducting composite materials, which alsoserve as current collectors. Gas-guiding channels are defined on a GDEfacing a flow field plate or, more typically, on a flow field platesurface facing a GDE. Reactants (e.g., H₂ or methanol solution) andreaction products (e.g., CO₂ at the anode of a DMFC, and water at thecathode side) are guided to flow into or out of the cell through theflow field plates. The configuration mentioned above forms a basic fuelcell unit. Conventionally, a fuel cell stack comprises a number of basicfuel cell units that are electrically connected in series to provide adesired output voltage. If desired, cooling channels and humidifyingplates may be added to assist in the operation of a fuel cell stack.

In one common practice, a fuel flow field plate and an oxidant gas flowfield plate are separately made and then assembled together to form abipolar plate (one side of a bipolar plate serving as a negativeterminal and the other side as a positive terminal, hence the name). Insome cases, an additional separator is sandwiched between the two flowfield plates to form a bipolar plate. It would be highly advantageous ifthe flow filed plates and the separator can be mass-produced into anintegrated bipolar plate assembly. This could significantly reduce theoverall fuel cell production costs and reduce contact ohmic lossesacross constituent plate interfaces. The bipolar plate is known tosignificantly impact the performance, durability, and cost of a fuelcell system. The bipolar plate, which is typically machined fromgraphite, is one of the most costly components in a PEM fuel cell.

Fluid flow field plates have open-faced channels formed in one or bothopposing major surfaces for distributing reactants to the gas diffuserplates (the anode and cathode backing layers, typically made of carbonpaper or fabric). The open-faced channels also provide passages for theremoval of reaction products and depleted reactant streams. Optionally,a bipolar plate may have coolant channels to manage the fuel celltemperature. A bipolar plate should have the following desirablecharacteristics: high electrical conductivity (e.g., preferably having aconductivity no less than 100 S/cm), low permeability to fuel or oxidantfluids, good corrosion resistance, and good structural integrity.

Conventional methods of fabricating fluid flow field plates require theengraving or milling of flow channels into the surface of rigid platesformed of a metal, graphite, or carbon-resin composite. These methods offabrication place significant restrictions on the minimum achievablefuel cell thickness due to the machining process, plate permeability,and required mechanical properties. Further, such plates are expensivedue to high machining costs. The machining of channels into the graphiteplate surfaces causes significant tool wear and requires significantprocessing times.

Alternatively, fluid flow field plates can be made by a laminationprocess (e.g., U.S. Pat. No. 5,300,370, issued Apr. 5, 1994), wherein anelectrically conductive, fluid impermeable separator layer and anelectrically conductive stencil layer are consolidated to form oneopen-faced channel. Presumably, two conductive stencil layers and oneseparator layer may be laminated to form a bipolar plate. It is oftendifficult and time-consuming to properly position and align theseparator and stencil layers. Die-cutting of stencil layers require aminimum layer thickness, which limits the extent to which fuel cellstack thickness can be reduced. Such laminated fluid flow fieldassemblies tend to have higher manufacturing costs than integratedplates, due to the number of manufacturing steps associated with formingand consolidating the separate layers. They are also prone todelamination due to poor interfacial adhesion and vastly differentcoefficients of thermal expansion between a stencil layer (typically ametal) and a separator layer.

A variety of composite bipolar plates have been developed, which aremostly made by compression molding of polymer matrices (thermoplastic orthermoset resins) filled with conductive particles such as graphitepowders or fibers. Because most polymers have extremely low electronicconductivity, excessive conductive fillers have to be incorporated,resulting in an extremely high viscosity of the filled polymer melt orliquid resin and, hence, making it very difficult to process. Bi-polarplates for use in PEM fuel cells constructed of graphite powder/fiberfilled resin composite materials and having gas flow channels arereviewed by Wilson, et al (U.S. Pat. No. 6,248,467, Jun. 19, 2001).Injection-molded composite-based bipolar plates are disclosed by Saito,et al. (U.S. Pat. Nos. 6,881,512, Apr. 19, 2005 and U.S. Pat. No.6,939,638, Sep. 6, 2005). These thermoplastic or thermoset compositesexhibit a bulk conductivity significantly lower than 100 S/cm (the USDepartment of Energy target value), typically not much higher than 10S/cm.

Besmann, et al. disclosed a carbon/carbon composite-based bipolar plate(U.S. Pat. No. 6,171,720 (Jan. 9, 2001) and U.S. Pat. No. 6,037,073(Mar. 14, 2000)). The manufacture process consists of multiple steps,including production of a carbon fiber/phenolic resin preform via slurrymolding, followed by a compression-molding step. The molded part is thenpyrolyzed at a high temperature (1,500° C.-2,500° C.) to obtain a highlyporous carbon/carbon composite. This is followed by chemical vaporinfiltration (CVI) of a carbon matrix into this porous structure. It iswell-known that CVI is a very time-consuming and energy-intensiveprocess and the resulting carbon/carbon composite, although exhibiting ahigh electrical conductivity, is very expensive.

Instead of using pyrolyzation and CVI to produce carbon/carboncomposites, Huang, et al. (U.S. Patent Application Pub. No.2004/0229993, Nov. 18, 2004) discloses a process to produce athermoplastic composite with a high graphite loading. First, polymerfibers, such as thermotropic liquid crystalline polymers or polyester,reinforcing fibers such as glass fibers, and graphite particles arecombined with water to form a slurry. The slurry is pumped and depositedonto a sieve screen. The sieve screen serves the function of separatingthe water from the mixture of polymer fibers, glass fibers and graphite.The mixture forms a wet-lay sheet which is placed in an oven. Uponheating to a temperature sufficient to melt the polymer fibers, thewet-lay sheet is allowed to cool and have the polymer material solidify.Upon solidification, the wet-lay sheet takes the form of a sheetmaterial with reinforcement glass fibers held together by globules ofthermoplastic material, and graphite particles adhered to the sheetmaterial by the thermoplastic material. Several of these sheets are thenstacked, preferably with additional graphite powder interspersed betweensheets, and compression-molded in a hot press. After application of heatand pressure in the press, one or more formed bipolar plates areobtained, where the bipolar plates are a composite of glass fibers,thermoplastic matrix and graphite particles. Clearly, this is also atedious process which is not amenable to mass production.

Alternatively, fluid flow field plates can be made from an electricallyconductive, substantially fluid impermeable material that issufficiently compressible or moldable so as to permit embossing.Flexible graphite sheet is generally suitable for this purpose becauseit is relatively impervious to typical fuel cell reactants and coolantsand thus is capable of isolating the fuel, oxidant, and coolant fluidstreams from each other. It is also compressible and embossing processesmay be used to form channels in one or both major surfaces. The“flexible graphite” is the exfoliated reaction product of rapidly heatednatural graphite particles which have been treated with an agent thatintercalates into the crystal structure of the graphite to expand theintercalated particles at least 80 or more times (up to 1000 times) inthe direction perpendicular to the carbon layers in the crystalstructure. The exfoliated graphite particles are vermiform inappearance, and are therefore commonly referred to as worms. The wormsmay be compressed together into flexible sheets which, unlike theoriginal graphite flakes, can be formed and cut into various shapes.These thin sheets (foils or films) are hereinafter referred to asflexible graphite. Flexible graphite can be wound up on a drum to form aroll of thin film, just like a roll of thin plastic film or paper.

Although highly conductive, flexible graphite sheets by themselves donot have sufficient stiffness and must be supported by a core layer orimpregnated with a resin. For example, U.S. Pat. No. 5,527,363 (Jun. 18,1996) discloses a fluid flow field plate comprising a metal sheetinterposed between two flexible graphite (FG) sheets having flowchannels embossed on a major surface thereof. These FG-metal-FGlaminates are also subject to the delamination or blistering problem,which could weaken the plate and may make it more fluid permeable.Delamination or blistering can also cause surface defects that mayaffect the flow channels on the plate. These problems may be difficultto detect during fabrication and may only emerge at a later date. Inparticular, thermal cycling between frozen and thawed conditions as arelikely to be encountered in an automobile application of the fuel cell,often results in delamination between a flexible graphite layer and themetal layer. Alternatively, Mercuri, et al. (U.S. Pat. No. 5,885,728,Mar. 23, 1999) discloses a flexible graphite sheet having embeddedfibers extending from its surface into the sheet to increase the resinpermeability of the sheet for the preparation of a resin-impregnatedflexible graphite bipolar plate. The step of adding ceramic fiberssignificantly increases the process complexity and cost.

The flow field plate should be constructed from inexpensive startingmaterials, materials that are easily formed into any plateconfiguration, preferably using a continuous molding process, andmaterials that are corrosion resistant in low temperature fuel cells andthat do not require further processing such as high temperaturepyrolization treatments. Any laminated or multi-layer plate should haveadequate bonding between layers to ensure structural integrity andreduced contact resistance (reduced power loss due to joule heating).

Accordingly, an object of the present invention is to provide a new andimproved fuel cell flow field plate or a bipolar plate that is awell-integrated sheet molding compound (SMC) component made by using afast and cost-effective process. The process can be automated andadaptable for mass production. In particular, the bipolar plate has theflexible graphite serving as the top and bottom sheets, which are bondedby an electrically conductive resin mixture. The resulting fuel cellsystem is highly conductive and well-suited to being a currentcollector.

Another object of the present invention is to provide a fuel cell flowfield plate that is a sheet molding compound composed of at least aflexible graphite sheet and an electrically conductive resinmixture-based separator layer that are well-bonded into a flow fieldplate, wherein the flexible graphite sheet has a surface flow fieldchannel.

SUMMARY OF THE INVENTION

One embodiment of the prevent invention is a sheet molding compound(SMC) composition, particularly for use as a fuel cell flow field plateor bipolar plate. The SMC composition comprises a top sheet, a bottomsheet, and a resin mixture sandwiched between the top sheet and thebottom sheet. At least one of the top sheet and bottom sheet comprises aflexible graphite sheet. The flexible graphite sheet has a planar outersurface having formed therein a fluid flow channel. The resin mixturecomprises a thermoset resin and a conductive filler present in asufficient quantity to render the SMC composition electricallyconductive enough to be a current collector material. When the resin iscured or solidified, the two sheets are well bonded by the resin toprovide good structural integrity to the resulting “laminated”structure.

When both the top and bottom sheets are flexible graphite, bonded by anelectrically conductive resin mixture, the resulting three-layer platecan be used as a bipolar plate that is interposed between two fuel cellunits. In this case, each flexible graphite sheet has a substantiallyplanar outer surface having fluid flow channels molded therein. Theseflow channels are preferably created through embossing during or afterthe SMC is made on a continuous basis.

If only one sheet (say, the top sheet) is flexible graphite and thebottom sheet is a sheet of plastic material (plastic film), the flexiblegraphite sheet and the plastic sheet may be laminated initially into athree-layer SMC plate. A mold release agent may be used between theplastic sheet and the resin mixture layer to facilitate later separationof the plastic sheet from the resin mixture-bonded flexible graphiteplate. Embossing or matched-mold pressing is carried out before, during,and/or after resin curing to produce flow channels on the outer surfaceof the flexible graphite sheet. The plastic sheet or film is then peeledoff, leaving behind a two-layer plate that can be used as a flow fieldplate.

Another embodiment of the present invention is a sheet molding compoundcomposition, comprising a top sheet, a bottom sheet, and a resin mixturesandwiched between the top sheet and the bottom sheet. The top sheetand/or the bottom sheet comprises a flexible graphite sheet. The resinmixture comprises a thermoset resin and a conductive filler present in asufficient quantity to render the resin mixture electrically conductivewith a bulk conductivity of the resin mixture (after curing) no lessthan 10 S/cm (preferably no less than 50 S/cm). The resultingthree-layer SMC composition (after resin curing or molding) has aconductivity typically above 100 S/cm, which is the US Department ofEnergy (DOE) target for composite bipolar plates. In many cases, the SMCconductivity exceeds 200 S/cm and, in some cases, exceeds 250 S/cm,which are quite impressive.

In the aforementioned SMCs, the conductive filler comprises a conductivematerial selected from the group consisting of carbon fibers, metalfibers, carbon nano-tubes, graphitic nano-fibers, nano-scaled grapheneplates, carbon blacks, metal particles, and combinations thereof Theconductive material being present in an amount of at least about 3% byweight (preferably at least 20% by weight), based on total weight of theresin mixture. Preferably, the SMC composition as defined above has aresin mixture having a thickness no greater than 1/15 of the sum of thetop sheet thickness and the bottom sheet thickness.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1: A sectional view of a prior art PEM fuel cell consisting of amembrane electrode assembly (MEA) sandwiched between two flow fieldplates 21, 23.

FIG. 2: A sectional view of a fuel cell stack consisting of two fuelcell units connected in series through a bipolar plate 19.

FIG. 3: A sectional view of (a) a bipolar plate consisting of a topflexible graphite layer, a bottom flexible graphite layer, and a coreresin-mixture layer; (b) a flow field plate consisting of a top flexiblegraphite layer, a core resin mixture layer, and a plastic film as atentative bottom layer; and (c) a flow field plate with the plastic filmpeeled off.

FIG. 4: (a) Schematic of a roll-to-roll process for preparing a highlyconductive sheet molding compound (SMC); (b) schematic of a process forfabricating SMC-based flow field plates or bipolar plates with thesurface flow channels being generated via in-line embossing ormatched-mold pressing.

FIG. 5: (a) Schematic of two matting flow field plates each with half ofthe coolant channels; (b) the two plates, after being molded with thethermoset resin cured, are combined to form a bi-polar plate withcoolant channels.

FIG. 6: (a) Schematic of two matting SMC laminates (prior to being fullycured) being molded in a matched-mold pressing process with molding pinsbeing inserted to produce coolant channels; (b) the resulting integralbipolar plate with built-in coolant channels.

FIG. 7: A sectional view of stacked fuel cells using a series of bipolarplates in accordance with the present invention.

FIG. 8: SMC bipolar plate conductivity as a function of toplayer-to-resin mixture layer thickness ratio for the first set ofexamples.

DETAILED DESCRIPTION OF THE INVENTION

A prior art fuel cell, as shown in FIG. 1, typically comprises amembrane electrode assembly 8, which comprises a proton exchangemembrane 14 (PEM), an anode backing layer 10 connected to one face ofthe PEM 14, and a cathode backing layer 12 connected to the oppositeface of PEM 14. Anode backing layer 10 is also referred to as a fluiddiffusion layer or diffuser, typically made of carbon paper or carboncloth. A platinum/ruthenium electro-catalytic film 16 is positioned atthe interface between the anode backing layer and PEM 14 for promotingoxidation of the methanol fuel. Similarly, at the cathode side, thereare a backing layer or diffuser 12 (e.g., carbon paper or carbon cloth)and a platinum electro-catalytic film 18 positioned at the interfacebetween the cathode backing layer and PEM 14 for promoting reduction ofthe oxidant.

In practice, the proton exchange membrane in a PEM-based fuel cell istypically coated on both sides with a catalyst (e.g., Pt/Ru or Pt) toform a catalyst-coated membrane 9 (CCM). The CCM layer 9 is thensandwiched between an anode backing layer 10 (diffuser) and a cathodebacking layer 12 (diffuser). The resulting five-layer assembly is calleda membrane electrode assembly 8 (MEA). Although some fuel cell workerssometimes refer to CCM as a MEA, we prefer to take the MEA to mean afive-layer configuration: anode backing layer, anode catalyst layer,PEM, cathode catalyst layer, and cathode backing layer.

The fuel cell also comprises a pair of fluid distribution plates (alsoreferred to as fluid flow field plates) 21 and 23, which are positionedon opposite sides of membrane electrode assembly 8. Plate 21, whichserves as a fuel distribution plate, is shaped to define fuel flowchannels 22 facing towards anode diffuser 10. Channels 22 are designedto uniformly deliver the fuel to the diffuser, which transports the fuelto the anode catalyst layer 16. An input port and an output port (notshown), being in fluid communication with channels 22, may also beprovided in flow field plate 21 so that carbon dioxide (in a DMFC) canbe withdrawn from channels 22.

Flow field plate 23 is shaped to include fluid channels 24 for passageof a quantity of gaseous oxygen (or air). An input port and an outputport (not shown) are provided in plate 23, which are in fluidcommunication with channels 24 so that oxygen (or air) can betransported through the input port to the cathode diffuser 12 andcathode catalyst layer 18, and water and excess oxygen (or air) can bewithdrawn from channels 24 through the output port. Plate 23 iselectrically conductive and in electrical contact with cathode diffuser12. It can be used as a uni-polar plate (the positive terminal of theelectrical current generated by the fuel cell unit) or as a part of abi-polar plate (if integrated with fuel flow field plate 21). Shown inFIG. 2 is a fuel cell stack that consists of two fuel cell units. On thetwo opposite sides of the stack are two separate flow field plates 21 a,23 a. Between the two MEAs (8 a and 8 b) is a bipolar plate 19, whichcan be viewed as two flow field plates integrated into one singlecomponent.

In the manufacture of fiber reinforced resin composite products,conventional (prior art) sheet molding compounds (SMCs) are frequentlyused which consist of a mixture of a viscous uncured thermosetting resinand chopped fibers, such as glass fibers. In most cases the resin andchopped fibers are sandwiched between films of plastic material to forma laminated structure which is wound in coiled form. The laminate isstored under conditions which will not result in final curing of theresin. At the time of use, the laminate is uncoiled and cut to thedesired size and shape for the molding operation. As the resin which isemployed to provide the sheet molding compound is relatively viscous,specific procedures must be employed to provide a thorough impregnationof fibers with the resin. Impregnation can be achieved by passing thelaminated structure between cooperating rolls or flexing the laminate inconcave and convex shapes. These prior art SMC composites do not have asufficient electrical conductivity for flow field plate or bipolar plateapplications.

The present invention provides a highly conductive sheet moldingcompound (SMC) composition and a fuel cell flow field plate or bipolarplate made from this SMC composition. The SMC-based bipolar plate,schematically shown in FIG. 3( a), comprises a top sheet 71, a bottomsheet 73, and a resin mixture 75 sandwiched between the top sheet andthe bottom sheet.

At least one of the top sheet and the bottom sheet comprises a flexiblegraphite sheet. The flexible graphite sheet (e.g., the top sheet 71) hasa planar outer surface 77 having formed therein a fluid flow channel 79.The resin mixture 75 comprises a thermoset resin (with or without acatalyst) and a conductive filler present in a sufficient quantity torender the SMC composition electrically conductive enough to be acurrent collector material (with a conductivity of the SMC preferably noless than 100 S/cm). When the resin is cured or solidified, the twosheets 71,73 are well bonded by the resin to provide good structuralintegrity to the resulting “laminated” structure. The thermoset resincan be any resin which, upon exposure to heat or high energy radiation(e.g., electron beam), becomes cured (e.g., forming a cross-linkednetwork). The thermoset resin may be advantageously selected from thegroup consisting of unsaturated polyester resins, vinyl esters, epoxies,phenolic resins, polyimide resins, bismaleimide resins, polyurethaneresins, and combinations thereof.

In one preferred embodiment (FIG. 3( a)), when both the top and bottomsheets are flexible graphite and are bonded by an electricallyconductive resin mixture, the resulting three-layer plate can be used asa bipolar plate that is interposed between two fuel cell units. In thiscase, each flexible graphite sheet has a substantially planar outersurface (e.g., surface 77 on the top sheet 71 and surface 81 on thebottom sheet 73) having fluid flow channels (e.g., channel 79 on the topsheet and 83 on the bottom sheet) molded therein. These flow channelsare preferably created through embossing during or after the SMC plateis made on a continuous basis.

The conductive filler in the resin mixture may be selected fromsmall-sized particles (preferably smaller than 10 μm and more preferablysmaller than 1 μm) such as a carbon black, graphite particle,nano-scaled graphene plate, graphitic nano-fiber, metal particle, or acombination thereof. When a thicker resin mixture layer (alsohereinafter referred to as the core layer) is allowed or desired, carbonor graphite fibers (fiber diameter typically greater than 12 μm) mayalso be used, alone or in conjunction with the aforementionedsmall-sized particles. A second thermoset resin or a thermoplastic maybe used to adjust the mixture viscosity and to assist in bonding thefiller particles together. Although not necessarily preferred, aquantity of other types of reinforcement fiber, such as glass fiber orpolymer fiber, may be added to impart additional structural integrity tothe resin mixture layer and that of the SMC.

The type and proportion of the conductive filler are preferably chosenin such a way that the bulk conductivity of the resulting resin mixtureis greater than 10 S/cm, further preferably greater than 50 S/cm, andmost preferably greater than 100 S/cm. Typically, when the conductivefiller proportion in the resin mixture is between 3% and 20% by weight(based on the total weight of the resin mixture), the bulk conductivityof the resin mixture exceeds 10 S/cm, up to approximately 35 S/cm,depending on the filler type. When the proportion is betweenapproximately 20% and 45%, the resin matrix conductivity exceeds 50S/cm. When the proportion is greater than 45%, the resin matrixconductivity exceeds 100 S/cm. This has led to SMC plates having anoverall conductivity mostly greater than 100 S/cm, typically greaterthan 200 S/cm, and, in many cases, even greater 250 S/cm, far exceedingthe US Department of Energy conductivity target (100 S/cm) for compositebipolar plates.

If only one sheet (say, the top sheet 71 a) is flexible graphite and thebottom sheet is a sheet of plastic material (plastic film 73 a), theflexible graphite sheet and the plastic sheet may be laminated initiallyinto a three-layer SMC structure (FIG. 3( b)). A mold release agent maybe used between the plastic sheet and the resin mixture layer tofacilitate later separation of the plastic sheet from the resinmixture-bonded flexible graphite plate. Embossing or matched-moldpressing is carried out before, during, and/or after resin curing toproduce flow channels 79 a on the outer surface 77 a of the flexiblegraphite sheet 71 a. The plastic sheet or film 73 a is then peeled off,leaving behind a two-layer plate (FIG. 3( c)) that can be used as a flowfield plate.

In one embodiment of the present invention, the top sheet is a flexiblegraphite foil, but the bottom sheet can be an electrically conductivefilm or foil, such as a carbon paper, carbon or graphite fabric,conductive polymer film, or metal foil. This will also make a goodbipolar plate. In another preferred embodiment, both the top and bottomsheets can be selected from a carbon paper, carbon/graphite fabric,carbon/graphite fiber-containing mat, conductive polymer film, thinmetal foil and/or flexible graphite. In theses cases, a portion of thethermoset resin in the resin mixture of the core layer can permeate intothe top or bottom layer to further enhance the structural integrity ofthe resulting laminate.

The overall conductivity of a two-layer flow field plate or athree-layer bipolar plate also depends upon the relative thickness ofthe resin matrix layer (or core layer) with respect to the totalthickness of the flexible graphite sheets. Since the flexible graphiteis highly conductive (typically with a conductivity greater than 300S/cm) and the resin matrix layer is typically lower than flexiblegraphite in conductivity, the resin matrix layer should be made as thinas possible to achieve a maximum electronic conductivity. When the resinmatrix conductivity is relatively low (e.g., 10 S/cm), a ratio of corelayer thickness-to-total flexible graphite thickness typically is assmall as 1/15 in order to achieve a bipolar plate conductivity of 100S/cm or greater. With both of the top and bottom layers being each 0.3mm (300 μm) thick, the resin mixture layer is thinner than 40 μm. Inorder to obtain a bipolar plate conductivity of 200 S/cm, the resinmixture layer has to be lower than 10 μm. However, a certain minimumcore layer thickness may be desired to obtain a desired level ofmechanical stiffness or strength of the bipolar plate.

By contrast, if the resin matrix conductivity is higher (e.g., 50 S/cm),a much higher ratio of core layer thickness-to-total flexible graphitethickness can be used in order to achieve a bipolar plate conductivityof 100 S/cm. In this case, the resin mixture layer can be almost asthick as a flexible graphite sheet. More advantageously, when eachflexible graphite sheet is approximately 300 μm thick and when the resinmixture layer is 60 μm or thinner, a bipolar plate conductivity as highas 200 S/cm can be achieved. A resin matrix layer of approximately 60 μmcan be prepared quite easily. The above observations will be furtherillustrated when examples are given at a later section.

Electrically conductive and corrosion resistant materials are useful forother applications (e.g., battery electrodes) than fuel cells. Hence,the presently invented SMCs have a wide range of applications. Thus,another embodiment of the present invention is a sheet molding compoundcomposition, comprising a top sheet, a bottom sheet, and a resin mixturesandwiched between the top sheet and the bottom sheet. The top sheetand/or the bottom sheet comprises a flexible graphite sheet. The resinmixture comprises a thermoset resin and a conductive filler present in asufficient quantity to render the resin mixture electrically conductivewith a bulk conductivity of the resin mixture no less than 10 S/cm(preferably no less than 50 S/cm). The resulting three-layer SMCcomposition, upon completion of resin curing to make a SMC product, hasa conductivity mostly above 100 S/cm, typically above 200 S/cm, and, inseveral cases, above 250 S/cm.

As indicated earlier, the conducting filler material may be selectedfrom carbon fibers, metal fibers, metal particles (preferablynano-scaled), carbon nano-tubes (CNTs), graphitic nano-fibers (GNFs),nano-scaled graphene plates, carbon blacks, or a combination thereof.Individual nano-scaled graphite planes (individual graphene sheets) andstacks of multiple nano-scaled graphene sheets are collectively callednano-sized graphene plates (NGPs). The structures of these materials maybe best visualized by making a longitudinal scission on the single-wallor multi-wall of a nano-tube along its tube axis direction and thenflattening up the resulting sheet or plate. These nano materials havestrength, stiffness, and electrical conductivity that are comparable tothose of carbon nano-tubes, but NGPs can be mass-produced at lowercosts. They can be produced by reducing the expanded graphite particlesto much smaller sizes (100 nanometers or smaller). The preparation ofother nano-scaled carbon-based materials, including CNTs, GNFs, andcarbon black, is well-known in the art. They are all commerciallyavailable, along with nano-scaled metal particles. These nano-scaled,electrically conductive filler materials are preferred conductive filleringredients for use in making the presently invented SMCs. It may befurther noted that CNTs, GNFs, and NGPs are known to be capable ofimparting high strength and stiffness to a resin matrix. They areideally suited for the present applications.

Referring to FIG. 4( a) as an example, the preparation of a flexiblegraphite SMC composition may begin with continuously or intermittentlyfeeding (uncoiling) a thin flexible graphite sheet 34 from a windingdrum 32. The surface of the flexible graphite sheet 34 may be coated (orpre-coated) with a desired layer 36 of an uncured thermoset resin via anumber of prior art coating techniques (e.g., spraying, printing,spin-coating, or, simply, brushing). A powder dispenser 38 is thenoperated to deposit a desired amount of a conductive filler 40 (orconductive filler plus some resin) onto the top surface of the thermosetresin layer 36 while the flexible graphite sheet is being driven forwardto the right. A leveling device 42 (e.g., a scraping blade) is used toform a uniform-thickness layer 39 of a resin-filler premix.Concurrently, another flexible graphite sheet 46, also coated orpre-coated with a thermoset resin layer 48 (with same or differentcomposition), is similarly fed from a drum 44 in such a way that theresin layer 48 comes in contact with the premix 39 to form a three-layerpre-SMC composition. This laminated pre-SMC composition is then fedthrough the gap between a pair of rollers 47 a, 47 b to compress thecomposition 50 a. A series of rollers (e.g., between 47 a, 47 b and 49a, 49 b) are used to assist in mixing of the resin with the conductivefiller. Specifically, impregnation or mixing of the fillerparticles/fibers with the resin can be achieved by passing the laminatedstructure 50 a between cooperating rolls or flexing the laminate inconcave and convex shapes to obtain a well-mixed SMC 50 b, which can bewound up on a roller 51. The SMC may be stored under conditions whichwill not result in final curing of the resin. A catalyst inhibitor maybe used to extend the shelf or storage life of the SMC without prematurecuring. When a flow field plate or bipolar plate is needed, the SMC isuncoiled and cut to the desired size and shape for the moldingoperation. Compression molding or hot pressing may be used to createflow channels on the outer surfaces of the plate while the thermosetresin is being cured and hardened.

Alternatively or preferably, as schematically shown in FIG. 4( b), acontinuous sheet of SMC is fabricated in a procedure similar to that inFIG. 4( a). Heating means may be used to advance the cure reaction ofthe thermoset resin (e.g., in a heating zone indicated by a phantom boxof FIG. 4( b)) to achieve a desired degree of curing before the SMC isembossed or matched-die molded between a pair of embossing tools 11 a,11 b or matting mold platens to create the desired flow field channels.These tools or mold platens may also be heated. As the laminated sheet(50 a or 50 b) continues to move forward, another portion of the sheetis embossed or molded. This is a continuous process that is suitable forcost-effective mass production of flow field plates or bipolar platesthat are highly conductive.

If one of the flexible sheets (either 34 or 46) is a plastic film, thisfilm may be peeled off after resin is cured to obtain a flow field plate(e.g., FIG. 3( c). If coolant channels are needed, they can be createdduring the SMC molding process in several ways. For instance, during theflow field plate molding process, the mold surface may be shaped toproduce a part of a channel groove (e.g., 52 a in FIG. 5( a)). Twomatting flow field plates may then be positioned together to form abipolar plate 54 (FIG. 5( b)) having coolant channels (e.g., 52).

Preferably, coolant channels are built into a bipolar plate when it ismolded. For instance, as schematically shown in FIG. 6( a), two uncuredor partially cured bi-layer SMC plates (with the plastic film peeledoff, leaving behind a resin mixture layer 63 a or 63 b and flexiblegraphite layer 65 a or 65 b) may be molded between a pair of matchedmolds (61 a, 61 b) and a number of molding pins 67. These pins, coatedwith a mold release agent, may be pulled out of the SMC structure toobtain an integral bipolar plate 54 (FIG. 6( b)) with built-in coolantchannels 67 a. Optionally, coolant channels may be fitted withconnectors, preferably before the resin matrix material is solidified.FIG. 7 shows back-to-back flow field plates that are fabricated as onemonolithic component 54, with coolant channels 52 formed as completechannels within the component, as well as reactant channels 60 & 62. Thetwo outer surfaces of bipolar plate 54 are stacked against respectivediffuser layers 56,58 (preferably made of carbon paper), which are inturn connected to catalyst-coated membrane (e.g., 70).

The present invention also provides a fuel cell or a stack of fuel cellsthat comprises a highly conductive flow field plate or bipolar platecomponent as defined in any of the aforementioned preferred embodiments.The resulting fuel cell system is of lower costs (due to theiramenability to mass production) and better performance (due to lowercontact resistance and internal resistance and, hence, higher voltage).

Conductivity measurements can be made by using the four-point probemethod on small-sized plate samples. Table 1 summarizes the parametersand properties of 28 samples prepared in the present study. The resultsshow that the core layer (resin mixture) composition and thickness havea profound influence to the conductivity of the resulting SMC bipolarplates. The thinner the core layer thickness, the higher the plateconductivity. Most of the SMC samples exhibit very impressive electronicconductivity.

TABLE 1 Properties of SMC bipolar plates (flexible graphite resistivity= 0.00333 Ω-cm). Core layer resin mixture Top layer Core layer CorePlate Sample composition thickness resistivity thickness conductivityNo. (Weight %) I₁ (cm) ρ₂ (Ω-cm) I₂ (μm) σ (S/cm) 1 65% Ep, 35% Ag 0.030.10 1.3 282 2 65% Ep, 25% Ag, 10% NGP 0.03 0.11 3.2 260 3 65% Ep, 25%Ag, 10% GNF 0.03 0.12 12 191 4 65% Ep, 34% Ag, 1% CNT 0.03 0.11 31 124 565% Ep, 20% CB, 15% NGP 0.03 0.13 55 87 6 65% Ep, 35% GP 0.03 0.11 78 697 65% Ep, 25% GP, 10% CF 0.03 0.12 120 51 8 65% Ep, 30% GP, 5% GL 0.030.14 220 34 9 65% Ep, 25% GP, 10% CF 0.03 0.12 310 27 10 35% Ep, 35% Ag,30% NGP 0.03 0.010 3.7 297 11 35% Ep, 55% Ag, 10% NGP 0.03 0.011 5.6 29412 35% Ep, 55% Ag, 10% GNF 0.03 0.012 15 286 13 35% Ep, 50% Ag, 15% CNT0.03 0.011 33 272 14 35% Ep, 40% CB, 25% NGP 0.03 0.013 45 263 15 35%Ep, 65% GP 0.03 0.011 75 245 16 35% Ep, 55% GP, 10% CF 0.03 0.012 160211 17 35% Ep, 60% GP, 5% GL 0.03 0.014 220 195 18 35% Ep, 55% GP, 10%CF 0.03 0.012 310 178 19 50% VE, 50% Ag 0.03 0.021 2.3 294 20 50% VE,40% Ag, 10% NGP 0.03 0.023 4.6 289 21 50% VE, 40% Ag, 10% GNF 0.03 0.02411 275 22 50% VE, 44% Ag, 1% CNT 0.03 0.023 29 243 23 50% VE, 30% CB,20% NGP 0.03 0.026 45 222 24 50% VE, 50% GP 0.03 0.023 63 203 25 50% VE,40% GP, 10% CF 0.03 0.024 88 183 26 50% VE, 45% GP, 5% GL 0.03 0.027 176140 27 50% VE, 40% GP, 10% CF 0.03 0.024 260 119 28 50% VE, 40% GP, 10%CF 0.03 0.024 320 109 Note: Ep = epoxy, VE = vinyl ester resin, GP =fine graphite particles, NGP = nano graphene plate, CB = carbon black,CF = carbon fiber, GL = glass fiber, GNF = graphitic nano-fiber, Ag =silver nano particles, CNT = carbon nano-tubes.

FIG. 8 further illustrates how the overall conductivity of a three-layerbipolar plate depends upon the ratio of the resin mixture layer (or corelayer) thickness to the thickness of a flexible graphite sheet (forSamples 1-9, the set of examples that have the highest resin mixtureresistivity). With the core layer resistivity slightly fluctuatingbetween 0.10 and 0.14Ω-cm, a top layer/core layer thickness ratio ofapproximately 7.5 or higher gives the 3-layer plate a conductivitygreater than 100 S/cm. With the top and bottom layer being each 0.3 mm(300 μm) thick, the resin mixture layer is 40 μm or thinner. In order toobtain a bipolar plate conductivity of 200 S/cm, the resin mixture layerhas to be lower than 10 μm (further preferably <5 μm). For other twosets of examples (Samples 10-18 and 19-28) given in Table 1, any toplayer/core layer ratio from 1/1 to 130/1 is good enough to result in ahigh bipolar plate conductivity (greater than 100 S/cm). Any ratiogreater than 5/1 leads to a plate conductivity greater than 200 S/cm.

These examples have clearly demonstrated the superior electricalconductivity of the presently invented SMC compositions and theSMC-based flow field plate or bipolar plate products. These conductivityvalues are far superior to those of most of prior art bipolar plates.

What is claimed is:
 1. A three-layer sheet molding compound for use as afuel cell flow field plate or bipolar plate, said composition beingcomposed of a top sheet, a bottom sheet, and a resin mixture sandwichedbetween said top sheet and said bottom sheet, wherein (a) at least oneof said top sheet and bottom sheet comprises a flexible graphite sheet;(b) said resin mixture comprises a thermoset resin and a conductivefiller present in a sufficient quantity to render said compositionelectrically conductive enough to be a fuel cell current collectormaterial and being not less than 10 S/cm, said conductive filler beingselected from the group consisting of carbon fibers, metal fibers,carbon nano-tubes, graphitic nano-fibers, nano-scaled grapheneplatelets, and combinations thereof, and the resin mixture is bonded to,and in direct contact with, the flexible graphite sheet; and (c) saidflexible graphite sheet has a planar outer surface having formed thereina fluid flow channel, and wherein the sheet molding compound has aconductivity that is not less than 100 S/cm.
 2. The sheet moldingcompound composition as defined in claim 1 wherein both said top sheetand bottom sheets are flexible graphite sheets having opposite planarouter surfaces, at least one of said outer surfaces having formedtherein a fluid flow channel.
 3. The sheet molding compound compositionas defined in claim 2 wherein each of said opposite outer surfaces hasformed therein a fluid flow channel.
 4. The sheet molding compoundcomposition as defined in claim 1 wherein said top sheet is a flexiblegraphite sheet having an outer planar surface having formed therein afluid flow channel, and said bottom sheet is not a flexible graphitesheet.
 5. The sheet molding compound composition as defined in claim 4wherein said bottom sheet comprises a carbon paper, carbon or graphitefabric, conductive polymer film, or metal foil.
 6. The sheet moldingcompound composition as defined in claim 4 wherein said bottom sheetcomprises a thermoplastic film which is removed before said flow fieldplate is incorporated as a fuel cell component.
 7. The sheet moldingcompound composition as defined in claim 1 wherein said thermoset resinis selected from the group consisting of unsaturated polyester resins,vinyl esters, epoxies, phenolic resins, polyimide resins, bismaleimideresins, polyurethane resins, and combinations thereof.
 8. The sheetmolding compound composition as defined in claim 1 further comprising acatalyst, inhibitor and/or a mold release agent.
 9. The sheet moldingcompound composition as defined in claim 1 wherein said composition hasan electrical conductivity no less than 200 S/cm.
 10. The sheet moldingcompound composition as defined in claim 9 wherein said conductivematerial being present in an amount of at least about 3% by weight,based on total weight of said resin mixture.
 11. The sheet moldingcompound composition as defined in claim 1, further comprising a coolantchannel.
 12. A fuel cell comprising a flow field plate made from a sheetmolding compound composition as defined in claim
 1. 13. A fuel cellstack comprising a plurality of fuel cell units separated by at least abipolar plate made from a sheet molding compound as defined in claim 3.14. The sheet molding compound as defined in claim 9, wherein said resinmixture is approximately 40 μm or thinner.
 15. The sheet moldingcompound as defined in claim 14, wherein said resin mixture isapproximately 5 μm or thinner.
 16. The sheet molding compound of claim14 wherein the filler comprises nano-scaled graphene platelets.